Gap junctions are plasma membrane specializations, present between most types of contacting cells in eukaryotes, through which the passive movement of small molecules from one cell to another occurs (direct cell-cell communication). Many studies have indicated that cell growth, development and differentiation may be regulated by the passage of small molecules through these junctions. This proposal is based upon a number of observations concerning the major poly peptide constituent of gap junctions isolated from liver. These include the demonstrated interaction of the isolated gap junction polypeptide with calmodulin, a calcium-dependent modulator protein which, by interacting with many cellular polypeptides, mediates calcium-dependent regulatory phenomena in vivo. The proposal includes in vitro experiments which will explore this and other interactions and modifications of the gap juncton polypeptide which may relate to the regulation of the junction channel. Evaluation of the physiological relevance of in vitro findings will be made in selected in vivo systems in which communication states can be manipulated. Antibody and affinity probes to gap junction polypeptides will be generated. Approaches are described to increase the perparative yield of the junction polypeptide to facilitate further experimentation. Attempts will be made to reconstitute gap junctional communication in vitro in a liopsome model system to permit direct analysis of the regulatory mechanisms of gap junctional communication. The information generated from the proposed studies should provide a basis for future examination of the molecules involved in communication, with particular emphasis on the role of gap junctional communication in the regulatory processes in which it has been implicated.